1. Field of the Invention
The present invention relates to a process for the production of aromatic carboxylic acids, in particular, to a process which permits to produce aromatic carboxylic acids of high quality by a catalyzed liquid/gas oxidation of alkyl aromatics, while eliminating the troubles due to foaming occurring on the liquid surface in the reactor.
2. Description of the Prior Art
There have been proposed processes for the production of aromatic carboxylic acids by oxidizing alkyl aromatic compounds, such as, paraxylene etc., in liquid phase by oxygen-containing gas in the presence of an oxidation catalyst, in which (1) the solvent vapor evaporated in the reactor is separated from the exhausted reaction gas by condensing it and the thus condensed solvent is recirculated by returning it to the reactor at its bottom (Japanese Patent Application Lay-Open No. 112044/1986 and the corresponding foreign Applications, namely, U.S. Pat. No. 4,593,122 and European Patent Application Lay-Open No. 125,341A) and in which (2) a predetermined proportion of the reaction gas delivered from the reactor is recirculated by turning it back into the reactor at a portion within the region of liquid phase (Japanese Patent Application Lay-Open No. 36439/1985 and the corresponding foreign Applications, namely, Belgian Pat. No. 90342, Brazilian Patent Application Lay-Open No. 8505367 and Spanish Pat. No. 8609191).
It had thus been recognized that lower reaction temperature and lower reaction pressure can be employed and a product of terephthalic acid (TPA) exhibiting higher light transmittance can be produced by recirculating the solvent or the reaction gas by turning them back to the reactor, in the manner explained above. Especially, in the above case (2), the oxidation reaction is effected with increasing oxygen partial pressure in the gas phase of the reaction system, whereby a reaction mixture containing a product of TPA having better light transmittance is obtained. It was reported that the hue of the polyester obtained was superior even though the content of 4-carboxy benzaldehyde (abbreviated hereinafter as 4-CBA) amounted to more than 500 ppm, when the product was subjected to an after-oxidation treatment, and that an ultrapure product having a light transmittance (T.sub.340) of some 95% could be obtained, when the reaction was carried out in such a condition that the content of 4-CBA amounted to 200-300 ppm.
In the above mentioned process (2), in which a predetermined proportion of the reaction gas delivered from the reactor is recirculated by turning it back to the reactor continuously at a portion within the liquid layer, although the advantageous effect suggested above can be attained, a considerable foaming on the liquid surface occurs. In a continuous production of an aromatic carboxylic acid by oxidizing an alkyl aromatic with oxygen-containing gas in liquid phase in the presence of a heavy metal compound and/or bromine-containing compound, the oxygencontaining gas is fed usually to the reactor at a portion within the region of liquid layer and the reaction is conducted under agitation, so that foaming occurs on the surface of liquid layer. The mist of reaction liqor containing the aromatic carboxylic acid suspended or dissolved therein, originated from collapsed foam will be entrained in the rising vapor and is carried over to the installation units, such as, heat exchanger, distillation tower and so on, in the succeeding process line, bringing about thereby problems, such as clogging of the line by the deposited carboxylic acid etc.
In the above prior process (2), the reaction gas after passing through the liquid layer in the reactor is recirculated by being returned to a portion in the region of liquid layer, so that such foaming problem is still more facilitated.
With the growing trend in recent years to the employment of large size units, the lifting velocitiy of the vapor in the gas layer at upper portion of the reactor becomes higher, so that troublesome problems caused by the foaming as mentioned above have grown to a matter of still greater significance. Therefore, it had become an inevitable technical requirement to solve the foaming problem for employing large size units.
Another important problem for effecting recirculation of a part of the reaction gas freed from the condensing components by returning it to a portion of the region of liquid phase in the reactor is a large power consumption by the gas recirculation blower. This constitutes also a substantial shortcoming in this process, since the gas compression energy required for overcoming the hydrostatic head by the liquid layer depth which has now become greater due to the employment of large size unit is considerably large.